MONDAY, May 11, 2015 (HealthDay News) -- Adding more olive oil or nuts to a Mediterranean diet -- one rich in fruits, vegetables, fish and whole grains and low in red meat -- may help keep your mind sharper as you age, a new study suggests.

The Spanish researchers found that seniors following such diets had greater improvements in thinking and memory than people who were simply advised to eat a lower-fat diet.

"You can delay the onset of age-related mental decline with a healthy diet rich in foods with a high antioxidant power, such as virgin olive oil and nuts," said lead researcher Dr. Emilio Ros, director of the lipid clinic at the Hospital Clinic in Barcelona.

"Because the average age of participants was 67 when the trial began, one can say that it is never too late to change your diet to maintain or even improve brain function," he said.

The report was published online May 11 in the journal JAMA Internal Medicine.

Dr. Sam Gandy, director of the Center for Cognitive Health at Mount Sinai Hospital in New York City, said, "The general heart-healthy and brain-healthy effects of eating less beef and more chicken, fish, fruits and vegetables has been validated to the point that I now recommend this general Mediterranean diet to all my patients."

Both olive oil and nuts have been associated with mental benefit in other studies, he added. "So, these findings are not so much a surprise as a reminder that there is more to the Mediterranean diet than meat, fruits and vegetables, and that calling out specific recommendations to include olive oil and nuts is probably worthwhile," Gandy said.

For the study, Ros and colleagues collected data on nearly 450 older adults between 2003 and 2009. Their average age was 67. All of the participants were at high risk for heart disease, but had no reported problems with memory or thinking.

Participants were randomly assigned to add a liter (about 33 ounces) of extra virgin olive oil per week to their Mediterranean diet, or to supplement their Mediterranean diet with 30 grams (roughly 1 ounce) per day of a mixture of walnuts, hazelnuts and almonds. Others followed a low-fat diet.

Mental changes over time were assessed with a battery of memory, attention and thinking tests. Complete data on almost 350 patients was available for analysis, the researchers said. The participants followed the diets for four years, on average, according to the study.

In both groups following a Mediterranean diet, the researchers saw improvements in tests of memory and thinking compared to the group on the low-fat diet, the study showed.

Samantha Heller, a senior clinical nutritionist at New York University Medical Center in New York City, explained that "healthy fats from foods like nuts and olive oil play crucial roles in brain function and health."

How might a Mediterranean diet that includes extra olive oil or nuts help? One way might be by protecting the nerve cells in the brain, suggested Heller, who was not involved with the study.

Every one of the nerve cells in the human brain is surrounded by an ultra-thin layer of fat and protein called the myelin sheath, she explained. The myelin sheath protects the nerve structure and helps nerve cell interaction. The brain gets its fats to make and maintain the myelin sheath from the foods people eat. The healthier the foods and fats, the healthier the brain, Heller said.

Oleic acid is one of the most prevalent fats in the myelin sheath, Heller said. "Olive oil, almonds, pecans, macadamias, peanuts and avocados are all good sources of this fat. Omega-3 fatty acids are also important for brain health and are found in fish, walnuts and soy foods. These healthy fats have been shown to improve mental function and brain health and reduce the risk of Alzheimer's disease," she said.

Heller suggested cooking with olive oil instead of butter, snacking on a handful of almonds instead of a bag of chips, and adding vegetables to pasta in place of meatballs.